Numerous conventional methods and devices are available for determining a flow property of fluid media, i.e., liquids or gases. The flow properties can be any physical and/or chemically measurable properties that qualify or quantify a flow of the fluid medium. In particular, the property can be a flow speed and/or a mass flow and/or a volume flow.
Below, the present invention is described in particular with reference to so-called hot-film air mass sensors, as described for example in Konrad Reif (pub.): Sensoren im Kraftfahrzeug (Sensors in the motor vehicle), 1st ed., 2010, pp. 146-148. Such hot-film air mass sensors are based, as a rule, on a sensor chip, in particular a silicon sensor chip, having a sensor membrane as measurement surface or sensor region over which the fluid medium can flow. Generally, the sensor chip includes at least one heating element as well as at least two temperature sensors that are for example situated on the measurement surface of the sensor chip. From an asymmetry of the temperature profile acquired by the temperature sensors, which profile is influenced by the flow of the fluid medium, a mass flow and/or volume flow of the fluid medium can be inferred. Hot-film air mass sensors are standardly fashioned as plug-in sensors that can be placed fixedly or exchangeably in a flow tube. For example, this flow tube can be an intake channel of an internal combustion engine.
A partial flow of the medium flows through at least one main channel provided in the hot-film air mass sensor. A bypass channel is fashioned between the inlet and the outlet of the main channel. In particular, the bypass channel is fashioned so that it has a curved segment for diverting the partial flow of the medium entering through the inlet of the main channel, the curved segment then going over into a segment in which the sensor chip is situated. The last-named segment is the actual measurement channel in which the sensor chip is situated. Here, in the bypass channel a means is provided that conducts the flow and counteracts a separation of the flow of the partial medium flow from the walls of the measurement channel. In addition, the inlet region of the main channel is provided, in the area of its opening oriented opposite the main flow direction, with oblique or curved surfaces that are formed such that medium flowing into the inlet region is diverted away from the part of the main channel that leads to the sensor chip. This has the effect that liquid or solid particles contained in the medium, due to their mass inertia, do not reach the sensor chip, and thus cannot contaminate it.
In practice, such hot-film air mass sensors must satisfy a large number of requirements and boundary conditions. Besides the goal of reducing a pressure drop at the hot-film air mass sensor overall through suitable flow-related designs, one of the main challenges is to further improve the signal quality and the robustness of such devices against contamination by oil and water droplets, as well as rust, dust, and other solid particles. This signal quality relates for example to a mass flow of the medium through the measurement channel leading to the sensor chip, as well as, if necessary, to the reduction of signal drift and the improvement of the signal-to-noise ratio. Here, signal drift refers to the deviation for example of the mass flow of the medium, in the sense of a change in the characteristic curve relation between the actually occurring mass flow and the signal that is to be outputted, ascertained in the context of calibration during manufacturing. In the ascertaining of the signal-noise ratio, the sensor signals outputted in a rapid temporal sequence are considered, whereas the characteristic curve or signal drift relates to a change in the mean value.
In standard hot-film air mass sensors of the type described, as a rule a sensor bearer, having a sensor chip attached or embedded thereon, extends into the measurement channel. For example, the sensor chip can be glued into or onto the sensor bearer. The sensor bearer can for example form a unit having a base plate made of metal, on which an electronics system, a control and evaluation circuit in the form of a circuit board, can also be glued. For example, the sensor bearer can be fashioned as an injection-molded plastic part of an electronics module. The sensor chip and the control and evaluation circuit can for example be connected to one another by bond connections. The resulting electronics module can for example be glued into a sensor housing, and the overall plug-in sensor can be sealed with covers.
Despite the improvements realized by this sensor system, there continues to be potential for improvement with regard to the precision of signal acquisition.
So that the hot-film air mass sensor can supply an air mass signal having as little interference as possible, it is important for there to be a flow that is as uniform as possible to the plug-in sensor, and through the measurement channel therein, and in particular over the measurement surface of the sensor chip. Between an end face of the sensor bearer and the wall of the measurement channel there is a gap whose width is subject to production-related fluctuations. In the region of the sensor bearer, the fluid medium flowing in the measurement channel is divided into three partial mass flows. A first partial mass flow flows over the sensor bearer and the sensor chip, a second partial mass flow flows under the sensor bearer, and a third partial mass flow flows through the gap. After the flow around the sensor bearer, an unstable wake forms having fluctuating flow speeds and pressures. This has the result that upstream as well, in particular in the region of the sensor chip, fluctuating flow quantities arise that cause fluctuations in the measurement signal, in particular having oscillation modes typical for the dimension of the sensor bearer and the flow speed.